Ruby has, at its root, an Object. Methods available in Object are available to every class because every class in Ruby inherits from Object somewhere in its own class hierarchy. Of course, you can override methods in subclasses, changing the functionality of a root method. You might stumble on to this idea if you work through Test Driven Development By Example by Kent Beck, translating the Java code into Ruby as you go. At some point pretty early on, he overrides the equality method on the Currency class to better test if two instances are equal. I'm going to do the same here, working with Instruments instead of Currency. Equality Equality in Ruby can be expressed using any of the following three methods object == other equal?(other) eql?(other) These methods are defined on the base Object. The default implementation of equality will only return true if both objects are ex...